A water sensor for detecting water in gas oil filters having a first and a second electrode a first and a second earth connection for earthing the first electrode and the second electrode; a first switch arranged in the first earth connection; a second switch arranged in the second earth connection; a first and a second current connection for injecting a first current into the first electrode and a second current into the second electrode; and a current-generating circuit connected to the first electrode and to the second electrode through the first current connection and the second current connection respectively, and designed to inject the first current into the first electrode and the second current into the second electrode, the first current and the second current being the same.